Stitch patterns used in sewing work of sewing machines include perfect stitches and hitch stitches. Perfect stitches are generally said to be normal stitches wavily made up of a needle thread and a bobbin thread. In the meantime, the hitch stitches are stitches that are formed when the needle thread is twisted on a per-stitch basis, thereby forming loops, and the bobbin thread is inserted into the loops.
In relation to the hitch stitches, difficulty is encountered in fastening the stitches uniformly by tightening the needle thread with a thread take-up lever, and slack again occurs in the stitches after the needle thread is tightened. Therefore, when the hitch stitches and the perfect stitches are mixedly woven, there arises a problem of stitch quality with uniform-spread stitches being hardly attained.
In order to prevent occurrence of the hitch stitches, Patent Documents 1 through 4 disclose rotating a sewing needle attached to a needle bar by rotating the needle bar around its axis of reciprocating motion.
For instance, in an automatic sewing machine disclosed in connection with Patent Document 1, a needle bar is rotated by a motor which rotates a needle bar drive unit, a spur gear which is rotated by the motor, and another spur gear, a drive shaft, still another spur gear, and an intermediate gear, etc. They transmit torque of the spur gear rotated by the motor successively.
In an automatic sewing machine disclosed in connection with Patent Document 2, a needle is rotated by a needle rotation motor.
Further, in Patent Document 3, a gear (a first gear) meshes with another gear (a second gear) fixed to a motor, and a needle bar slidably fitted into a groove of the first gear is rotated by rotation of the motor.
As illustrated in FIG. 27, in relation to a needle bar rotation mechanism of a sewing machine described in connection with Patent Document 4, a hole is opened in each of parallel pieces of a C-shaped receiving element 505a in a needle bar connecting stud 505. A sleeve 506 is inserted into the C-shaped space of the needle bar connecting stud 505, and a needle bar 508 penetrate through the holes and a hole of the sleeve 506. A screw 507 is inserted into a screw hole opened in the sleeve 506, and the sleeve 506 and the needle bar 508 are fastened by the screw 507. The needle bar 508 is rotatably held in a vertically-movable manner by braces 509a and 509b attached to a needle bar supporter 509. A cam groove 508a is formed in the needle bar 508. A needle bar guide pin 510 fixed to the brace 509a is engaged with the cam groove 508a. With this configuration, the needle bar 508 is thereby moved in the vertical direction while being rotated by means of vertical movement of the needle bar connecting stud 505.